Deprecate mismatched collation comparison for uniquness validator

In MySQL, the default collation is case insensitive. Since the
uniqueness validator enforces case sensitive comparison by default, it
frequently causes mismatched collation issues (performance, weird
behavior, etc) to MySQL users.

https://grosser.it/2009/12/11/validates_uniqness_of-mysql-slow/
https://github.com/rails/rails/issues/1399
https://github.com/rails/rails/pull/13465
https://github.com/gitlabhq/gitlabhq/commit/c1dddf8c7d947691729f6d64a8ea768b5c915855
https://github.com/huginn/huginn/pull/1330#discussion_r55152573

I'd like to deprecate the implicit default enforcing since I frequently
experienced the problems in code reviews.

Note that this change has no effect to sqlite3, postgresql, and
oracle-enhanced adapters which are implemented as case sensitive by
default, only affect to mysql2 adapter (I can take a work if sqlserver
adapter will support Rails 6.0).

1 files changed, 17 insertions, 10 deletions

diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 1dfea0f506..5d46bd47d9 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -8,6 +8,13 @@ ActiveRecord::Schema.define do
   #                                                                     #
   # ------------------------------------------------------------------- #
 
+  case_sensitive_options =
+    if current_adapter?(:Mysql2Adapter)
+      { collation: "utf8mb4_bin" }
+    else
+      {}
+    end
+
   create_table :accounts, force: true do |t|
     t.references :firm, index: false
     t.string  :firm_name
@@ -266,7 +273,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :dashboards, force: true, id: false do |t|
-    t.string :dashboard_id
+    t.string :dashboard_id, **case_sensitive_options
     t.string :name
   end
 
@@ -330,7 +337,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :essays, force: true do |t|
-    t.string :name
+    t.string :name, **case_sensitive_options
     t.string :writer_id
     t.string :writer_type
     t.string :category_id
@@ -338,7 +345,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :events, force: true do |t|
-    t.string :title, limit: 5
+    t.string :title, limit: 5, **case_sensitive_options
   end
 
   create_table :eyes, force: true do |t|
@@ -380,7 +387,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :guids, force: true do |t|
-    t.column :key, :string
+    t.column :key, :string, **case_sensitive_options
   end
 
   create_table :guitars, force: true do |t|
@@ -388,8 +395,8 @@ ActiveRecord::Schema.define do
   end
 
   create_table :inept_wizards, force: true do |t|
-    t.column :name, :string, null: false
-    t.column :city, :string, null: false
+    t.column :name, :string, null: false, **case_sensitive_options
+    t.column :city, :string, null: false, **case_sensitive_options
     t.column :type, :string
   end
 
@@ -876,8 +883,8 @@ ActiveRecord::Schema.define do
   end
 
   create_table :topics, force: true do |t|
-    t.string   :title, limit: 250
-    t.string   :author_name
+    t.string   :title, limit: 250, **case_sensitive_options
+    t.string   :author_name, **case_sensitive_options
     t.string   :author_email_address
     if subsecond_precision_supported?
       t.datetime :written_on, precision: 6
@@ -889,10 +896,10 @@ ActiveRecord::Schema.define do
     # use VARCHAR2(4000) instead of CLOB datatype as CLOB data type has many limitations in
     # Oracle SELECT WHERE clause which causes many unit test failures
     if current_adapter?(:OracleAdapter)
-      t.string   :content, limit: 4000
+      t.string   :content, limit: 4000, **case_sensitive_options
       t.string   :important, limit: 4000
     else
-      t.text     :content
+      t.text     :content, **case_sensitive_options
       t.text     :important
     end
     t.boolean  :approved, default: true